The present invention relates to a three-phase electric motor using high-efficiency permanent magnets having a low detent torque, in particular for high-speed drive applications in the industrial or automotive field.
The requirements in terms of bulk and efficiency of electric motors are leading to the use of high-energy magnets for the production of the rotors and the more the energy of the materials employed is increased, the more difficult it becomes to control the residual torques of the these motors. The cost of these motors depends not only on the costs of the materials employed but also on the costs of the methods used to produce the various elements. It is therefore important to implement simple solutions for the production of the spare parts and for the assembly of the various components of these motors. In particular, the control of the residual torques may lead to forms of stators that are incompatible with the more economical winding methods. Furthermore, the use of these motors at high speeds requires the use of a small number of pairs of poles for the rotor so as to reduce the losses in the motor and in the electronics.
FIGS. 5 and 6 describe motors of the prior art that make it possible to obtain low residual torques with rotors having a small number of pairs of poles. The motor of FIG. 5 uses a rotor with two pairs of poles and a stator with 6 teeth (91). The stator poles occupy 90% of the available surface area and thus make it possible to obtain a low residual torque. The small notch width results in a long and difficult winding, which is therefore costly and also has a poor bulk factor.
The motor of FIG. 6 also uses a rotor with two pairs of poles and, compared to the motor of FIG. 5, each stator pole is divided into three teeth. A stator winding then surrounds three teeth of the stator to form a single pole. The 18 stator teeth occupy 70% of the available surface area and thus make it possible to obtain a low residual torque, but require an interleaved winding of the three phases, which is therefore long and costly, and ill-suited to very large mass production.
Also known from the state of the art is the use of rotors that exhibit a sinusoidal induction and that, as a result, make it possible to reduce the residual torques. However, to obtain residual torque values such as those demanded by most industrial or automotive applications, the induction of these rotors must exhibit a very low distortion. The harmonic breakdown of the induction of these rotors must show very low percentages for harmonic 3, harmonic 5 and harmonic 7, in particular for the last two. In particular, by using high-energy magnets, the percentages of harmonic 5 and of harmonic 7 must be less than 0.5%. If the percentages of harmonics 5 and 7 are of the order of, or greater than, 0.5%, it is necessary to combine these rotor solutions with a stator solution that is appropriate in terms of the reduction of the residual torques. The motors of FIGS. 5 and 6 sometimes use rotors that exhibit a sinusoidal induction.
The present invention aims to remedy the drawbacks of the state of the art by proposing a motor structure which is particularly appropriate for very large volume manufacture, and suited to magnets with very high energy, using a small number of pairs of poles, typically 2, a reduced number of windings, typically 3, and exhibiting a very low residual torque as well as high performance levels at high speed. The present invention proposes a robust motor, exhibiting a low detent torque, excellent dynamic performance levels, and that is particularly economical by virtue of a simple structure for the stator and for the rotor. The motor of the invention is more specifically defined by claim 1. Other features, optional but advantageous, are defined in the claims that are directly or indirectly dependent on claim 1.
To this end, the invention relates to a three-phase motor formed by a stator part excited by electric windings and by a rotor with N pairs of poles, the stator part having teeth extending radially from an annular gear. The width of the stator poles is determined on the one hand to avoid any saturation in the tooth, and on the other hand to cancel the effect of the main magnetization harmonics and also so as to allow for an economical winding by allowing for the placement of windings produced outside the stator.
The stator poles are straight teeth which comprise an arc of circle (17) at their end defining an airgap at the center of the tooth and an airgap which can be enlarged or reduced on each of the sides (18 and 19). This means that the end of the tooth can be of convex or concave shape. The width of the stator pole, which is defined as being the angle at the center which encompasses the whole tooth, and the radius of the arc of circle (17) which forms the end of the tooth, are determined in such a way as to obtain, for each tooth, a residual torque that is as low as possible. The width of the tooth forming the core of the winding is defined according to the maximum energy of the magnets used. This energy can range from 6 to 40 MGOe, but the stator poles represent, in all cases, less than 60% of the available surface area. Thus, ease of winding is guaranteed.
As FIG. 12 shows, if the geometry of the end of the tooth is adapted according to the energy of the magnets and according to the magnetization harmonics, it still always observes one and the same rule which makes it possible to define an angle alpha (24) which is tangential to the sides (18 and 19) of the stator pole. Since the cancellation of the residual torque is obtained for each of the teeth individually, there is no need to seek a torque compensation between two teeth or a group of teeth as in certain motors of the prior art. New combinations between the number of teeth of the stator and number of pairs of poles of the rotor become useable while retaining all the advantages in terms of residual torque, motor torque constant and ease of industrialization and therefore represent a better trade-off in terms of cost and performance, in particular for high-speed drive applications.